>Perhaps a longer message is warranted - but make it standard (so you only have to copy and paste), make clear it is your standard message, and give sufficient reasons (individual attention takes too much time, in the case of public messages, other people can profit from the exchange, etc.)

Yes, I've done that with a "signature" in Outlook in order to make things easier... anyway, there was a guy who replied like this: "Hey, your answer seems like typed by your secretary who are trained to answer like that because you don't have good will to help people. You arrogant.". Guess how happy I was by reading that. :)